I am very sorry to hear about what you are facing here.
His progressive signs are suggestive of spinal based issues. Specifically, with the pain and deterioration of his hind limb function, we’d be concerned about a problem with one of his lower intervertebral discs. These are the shock absorbers between the vertebral bones. When they become diseased (ie inflamed, rupture, pushed out of the place and into the spinal canal), this can cause severe pain and the signs are reporting. Other potential issues that could appear this way, though are less common and less likely would be a spinal abscess, a spinal tumor, or possible fracture of the vertebrae putting pressure on the spine in its canal).
In this case, his prognosis is very guarded. It sounds like your vet already tried to treat for disc and spinal cord inflammation, but the deterioration and lack of response suggests one of these bigger issues. And the problem is that often if we don't get in there and remove the problem disc, they don't get better and can end up paralysed (its the same for people). So, its a less then ideal situation. Especially since while we can xray to check for vertebral bone fractures and may even be able to see disc changes (if they are calcified enough); we usually do have to use MRI to see through that bone and into the spinal canal. And that is quite important in cases like this since we need to know which disc or discs are in trouble so that the surgeon can remove them for him.
Overall, this is a terrbile situation for your lad. It sounds like we have one of these more serious issues lurking and really would need an MRI (we can try xray and hope we get lucky) to see where the disc is in trouble so it can be removed. Otherwise, all we can really do is speak to his vet about trying stronger pain relief, steroids, and muscle relaxants to at least keep the inflammation and pain under control for him.
Please take care,